Spatial music projecting device



April 8, 1953 P. H. SCHAEFFER 2,636,943

SPATIAL MUSIC PROJECTING DEVICE Filed Feb. 15. 1952 2 1 Patented Apr.28, 1953 ,UNITED STATES PATENT OFFICE v SPATIAL MUSIZfiSifiECTING DEVICEApplication February 13, 1952, Serial No. 271,335 In France February 26,1951 2 Claims. 1

The present invention relates to a device for projecting spatial musicand, more particularly, to a device for causing to be described, for avirtuai source of music, which'is'equivalent to four actual stationarysources or" music, which are fed at variable levels by the same musicalsignal, any desired trajectory in a three-divisional space.

The object of thepresent invention is-to provide a device of this kindin which the means for controlling the levels of'the stationary sourcesof music, the composition of which determines the location of thevirtual source, is a singlemanipulata'ble member; which can bemanipulated by an operator, like a conductorof an orchestra, in themanner 'of' a batch for the purpose of-obtain'ing artistic effects froma variable localisation of the said virtual source.

Another object of the .invention is to provide a device of'this kind inwhich the trajectory described by the virtual source of music issufficiently similar to the trajectory described by the single means forcontrolling the levels of the ,stationary sources of music.

The originality of the present invention as compared with thestereophonic methods of the prior art resides in'two essential points. i

The device or the invention does not aim at reinstating, in its threedimensions, an actual source of sound which occupies a certain volumebut to create a virtual source of sound which is localised at a point ofa three-dimensional space in which the listener is situated, thislocalisation 'iheing arbitrary and fixed by an operator; this adds .aspatial characteristic, called static spatialisation, to the audition ofthe music.

In addition, the device of ,t'heinvention enables an operator to applyto the virtual source, arbitrary movements'on selected trajectories;this adds, to the audition oi the music, a new characteristic calledcinematic spatialisation.

In the present invention, a correspondence ;between the points or atetrahedral volume hereinafter called control volume and the points of atetrahedral volume hereinafter called hearing volume is established, thehearing volume being a tetrahedron defined by four loud-speakers whichoccupy its apices and the-control volume-being 'a tetrahedron which isapproximately similar to the torm-ertetrahedron :and is defined byfourstationary coils that occupy its 'a pices; A movable "coil, which is,fed by-an alternating current of supersonic frequency, is held in thehand of an operator and can be displaced inside the control volume. Itinduces "voltages in the stationary coils. These voltages are "detectedand serve to control the gain of four amplifiers which respectively feedthe loud-speakers in such a. manner that the gain of each amplifier issubstantially inversely proportional to the dis- :tance of the saidmovable coil from the station ary coil, the induced voltage of whichcontrols this amplifier.

Under these conditions a correspondence which is substantially near tohomology is obtained between the position of the movable .coil in thecontrol volume and the position .of the virtual sourcelof music in thehearing volume.

()ther objects and features of the invention will appear on reading thefollowing detailed description with the aid of the accompanyingdrawings, in which:

Fig. 1 represents diagrammatically a concert hail, showing thearrangement in relation to .a "hearing space of four loudespeakers whichmark the boundaries of the hearing volume;

Fig. ,2 represents the movable coil, the stationary coils marking thebmmdanies .of the con? trol volume and the electric circuits controllingthe sound levels of the loud speakers;

Fig. Step-resents diagrammatically the sound level of a loud'speaker asa function of the distance between the movable coil and the station.-.ary coil, the current induced in which controls the said level; and

Fig. 4 shows the-means tor suspending the movable coil when thelat-teris not :being held in the hand.

Referring to Fig. 1, I represents diagrammatical ly a concert hall, thereverberation time of which is less than 0.5 second and the dimensionsof which are-definitely greater than those of the tetrahedral hearingvolume. In this hall, there are arranged four loud-speakers .2, -3, 4and i, the first three of which are arranged at the vertices of anequilateral triangle having a length of side of :a and at a height, fromthe soundabsorbing floor of the hail, of between 1 and 2 metres. Thefourth loud-speaker is arranged on the perpendicular passing through thecentre of theegui-lateral triangle tor-med by the three firstrnen-tionedloudspeakers and at a distance from the plane, which passes through thethree mentioned loud-speakers, of between and . 3 The listeners areplaced inside an almost circular curve 6 which is centred at the foot ofthe perpendicular 7, and the diameter of which is of the order of Theaxes of the loud-speakers are all directed to the centre 0 of thetetrahedron the apioes of which they define and they have apolar-radiation curve possessing an angle of opening of the order of 60.through the connections to 29 respectively.

Referring to Fig. 2, 8 denotes a reading system of a recording mediumwhich feeds-a pre-am- These loud-speakers are fed fhaving its middlepoint connected to earth and the cathodes of which are loaded by theresistance 56 and the potentiometer 51.

The sliding contact 58 of the potentiometer 51 is connected to the gridof the tube 59 of the amplifier H] which, in addition, receives thesigplifier 9, the outlet of which is connected to fouramplifiers H], II,I2 and [3. These four amplifiers comprise a second inlet to which isapplied a signal for controlling the gain by means of theconnections l4,l5, l5 and I1 respectively.

I The amplifiers [0 to E3 are followed by expander circuits 8 to 21which are themselves followed by power amplifiers 22 to 25. The outletsof these power amplifiers are connected to the loud-speakers 2 to 5 byconnections 23 to 29 respectively.

The signals for controlling the gain of the amplifiers [0 to I3 areobtained in the following manner:

Mounted on four supports 30 to 33 are four I,

stationary coils 34 to 31, each having a diameter of the order of 50cms. and comprising about 500 turns of wire. These coils are fixed ontheir supports by means of a sliding arm 38 so as to form, around anoperator, a tetrahedron which i is 'approximately similar to thetetrahedron which is formed by the loud-speakers and the length of aside of which is of the order of 1.50 metres. The normals to the coilsconverge to the centre 0' of the tetrahedron.

A coil 40 (Fig. 4) may either be held in the hand of an operator or befixed on an arm 4| which slides on a second arm 42 that ends in aball-and-socket joint 43 carried by a support 44 and situated nearly atthe centre of the tetrahedron formed by the stationary coils. This coilis fed by a source 45 of alternating voltage, which has a frequency ofthe order of 5000 cycles per second, by means of connections 46. Itcomprises a divided iron core having the shape of a hollow cylinder 41with a diameter of 2 cms., and a length of 10 cms. terminated by asubstantially spherical widened" portion 48 having a diameter of 5 cms.The cylindrical part carries a coil 49 with about 1000 turns. Thevoltage at the terminals of the coil is about 1 volt. The coil radiatesva magnetic field, the lines of force of which are very markedly radialwith respect to the centre of the sphere 48, and it induces, in eachstationary coil, a voltage of a magnitude dependingupon its distance inrelation to the tationary coil concerned and which is substantiallyindependent of its orientation in relation to this stationary coil.

Connected to the terminals of each stationary coil by connections is apotentiometer 50, the sliding contact 52 of which is connected to thegrid of an amplifying tube 5! which is followed by a rectifying stage53. The outlets of the rectifying stages 53 are connected respectivelyby means of connections, l5, l5 and I! to the gain-control terminals ofthe amplifiers 10, ll, 12 and H3. The composition of a rectifying stage53 is given only for one {of the four stages, the three others beingidentical, and, in the same nal issuing from the pre-amplifier 9. Thetube 59 is a tube having a gain which is inversely proportional to thenegative polarisation voltage of its grid and therefore to the potentialof the sliding contact 58. There may be used, as the tube 59, a tube ofthe EF9 type for example or,

failing this, a tube of the BL? type.

The operation of the device for projecting spatial music is as follows:

If only two loud-speakers, 2 and 3 for example, are fed by musicalsignals of equalintensity, a listener situated inside the space 6 willperceive 'a virtual source of music situated at the centre .of thestraight line segment connecting the two loud-speakers. If the soundlevel of one of the loud-speakers in relation to the other exceeds acertain threshold value, the virtual source will leave the middle pointand will be displaced over the straight line segment joining the twoloudspeakers towards the one that is emitting the higher intensity ofsound. If the unbalance between the levels increases, the virtual sourcecontinues to approach the more intense source of sound and, if theunbalance reaches a sufficiently high limiting value, there is anabsolute preeminence of the loud-speaker emitting the greater intensityof sound with which the virtual source appears to coincide.

The law of variation of the angular direction of the virtual source foran observer situated approximately on the mediatrix of thesegmentjoining the two actual sources is rather complicated. It has beenstudied by numerous authors in the case of actual sources emitting puresounds, as a function of the frequency of these sounds. I have foundthat the threshold from which one would perceive respectively adisplacement, which is just perceptible, of the virtual source inrelation to the medial plane of the two actual sources and the limitfrom which a complete pre-eminence of one of the actual sources over theother took place were smaller in the case of musical sounds than in thecase of pure sounds. In the case of musical sounds, these thresholds andthese limits are respectively of the order of 4 and 8 decibels whilst,for pure sounds of 500 cycles per second, they are approximately 9 and26 decibels, for pure sounds of 1000 cycles per second, 13 and 30decibels and, for pure sounds of 2000 cycles per second, 20 and 35decibels. In practice, in order to be able to keep the possibility ofdisplacing, if not over the whole segment joining the actual sources, atleast over a part of that segment, a virtual source that is emittingboth pure sounds and complex sounds, it is necessary to allow oneself amargin of variation of gain of the amplifiers [0 to [3 which I havefound to be of the order of 20 decibels.

What has hereinbefore been said for a relai reve t en 9 tha p si so m lns-j speakers 2 and 3 may be repeated for any couple of loud-speakersand, on extending the reasoning to the set of four loud-speakers, it isseen that, to every set of four sound levels given for the saidloud-speakers, there corresponds a virtual source having a givenlocation.

Each of the stationary coils 34 to 3l has current induced therein by themagnetic field produced by the movable coil 40. The induced voltages arerectified by the rectifier stages 53 and there is obtained, between theearth and the sliding contact 58 of each stage, a D. C. voltage which isa function of the distance of the movable coil 40 from the correspondingstationary coil. This voltage is negative and increases when thisdistance decreases. The amplifier I!) has a gain which is proportionalto the D. C. polarisation voltage of the grid in relation to the cut-offvoltage and, consequently, inversely proportional to the distancebetween the stationary coil and the movable coil.

The positions of the sliding contacts 52 and 58 and the values of theresistances of the potentiometers 50 and 5! are adjusted so that thegain of the amplifier corresponding to a given stationary coil is at themaximum when the movable coil is in the immediate vicinity of thisstationary coil cms. for example) and is at the minimum and nearly zerowhen the movable coil is near another stationary coil, that is to say ata distance of the order of 1.50 metres from the stationary coilconcerned.

The sound level L of a loud-speaker, as a function of the distance inmetres between the movable coil and the stationary coil controlling thisloud-speaker, is given by Fig. 3.

The amplifiers ill to Hi have an hyperbolical characteristic and theresult of this is that the dynamics of the amplified signal vary withthe actual gain of the amplifiers in relation to the dynamics of theinput signal. The expander circuits [8 to 2| are also controlled by therectified signals in the rectifier stages 53 and their object is to makethe dynamics of the amplified signals equal to the dynamics of the inputsignals.

The sound levels of the four loud-speakers are thus almost inverselyproportional to the mutual distances between the movable coil and thefour stationary coils. As the location of the virtual sound sourceinside the tetrahedron formed by the loud-speakers is a function ofthese sound levels and is nearer a given loudspeaker the higher thesound level of the latter,

there is a biunivocal correspondence between the location of the movablecoil and the virtual source of sound. In particular, if the movable coilis at the centre of the tetrahedron formed by the stationary coils, theloud-speakers 2 to 5 will emit equal intensities and the virtual sourcewill be located at the centre of the audience space 6 and, in practice,owing to a psychological efiect, the source will appear to surroundcompletely a listener located in the audience space.

If the movable coil 40 is held in the hand of the operator, the latterwill be able to displace it and his gestures will have the effect ofcausing the virtual source to describe any desired trajectory and thusto add a spatial stereophonic element to the audition of the music.

If the virtual source of music is to be localised at a fixed point ofthe hearing volume, the position of the movable coil 40 will be adjustedon the arm 32 by means of the clamping screw GI and the orientation ofthe movable arm 42 will be adjusted by means of the clamping band 39 ofthe ball-and-socket joint 43.

Although the invention has been described in respect to a completeexample of embodiment, it is to be understood that modifications arepossible and that the scope of these modifications is limited only bythe contents or" the appended claims.

What I claim is:

l. A device for projecting spatial music, com prising four loud-speakerssituated in a concert hall at the apices of a tetrahedron, fouramplifiers feeding the said loud-speakers, four stationary coilssituated at the apices of a smaller tetrahedron similar to thetetrahedron formed by the loud-speakers, a coil fed by an alternatingcurrent, movable inside the tetrahedron formed by the four stationarycoils and inducing a current in the said stationary coils, rectifiersconnected to the outlet of the stationary coils, and means forcontrolling the gain of the amplifiers proportionally to the D. C.voltages rectified by the said rectifiers, whereby the sound levels ofthe loud-speakers are substantially inversely proportional to thedistances of the movable coil from the stationary coils and the locationof the virtual sound source, which is equivalent to the fourloud-speakers, in relation to the tetrahedron formed by theloud-speakers is substantially homologous to the location of the movablecoil in relation to the tetrahedron formed by the stationary coils.

2. A device for projecting spatial music, comprising four loud-speakerssituated in a concert hall at the apices of a tetrahedron, fouramplifiers feeding the said loud-speakers, four stationary coilssituated at the apices of a smaller tetrahedron similar to thetetrahedron formed by the loud-speakers, a spherical polar terminationcylindrical core movable inside the tetrahedron formed by the fourstationary coils, a. coil mounted upon said core, fed by an alternatingcurrent and inducing a current in the said stationary coils, rectifiersconnected to the outlet of the stationary coils, and means forcontrolling the gain of the amplifiers proportionally to the D. C.voltages rectified by the said rectifiers, whereby the voltage inducedby the coil mounted upon the movable core in the stationary coilsdepends solely on the distance between said movable and stationary coilsand is substantially independent from the relative orientation of thetwo coils, the sound levels of the loudspeakers are substantiallyinversely proportional to the distances of the movable coil from thestationary coils and the location of the virtual sound source, which isequivalent to the four loud-speakers, in relation to the tetrahedronformed by the loud-speakers is substantially homologous to the locationof the movable coil in relation to the tetrahedron formed by thestationary coils.

PIERRE H. SCI-IAEFFER.

Name Date De Boer Sept. 13, 1949 Number

